Feedthrough capacitor electromagnetic interference (EMI) filters are well known in the art for decoupling and shielding of signals that are picked up by lead wires implanted in human body tissue. For example, in cardiac pacemaker applications, the wires that lead from the pacemaker to the heart often act as antennas and pick up stray electromagnetic interference from sources such as cell phones, RF identification systems, cell phone boosters, cell phone jammers, microwave ovens, and other emitters that are typically found in the patient environment. Another such powerful emitter is the RF field produced during magnetic resonance imaging (MRI). These signals often propagate into a pin and are then coupled into the interior of the active implantable medical device.
Common preferred practice in accordance with U.S. Pat. No. 5,333,095 and others is to mount a feedthrough filtered capacitor at the point of ingress and egress of the lead wires into an active implantable medical device (AIMD). Exemplary prior art EMI filters are shown and described in U.S. Pat. Nos. 5,333,095; 5,751,539; 5,896,267; 5,905,627; 5,959,829; 5,973,906; 5,978,204; 6,008,980; 6,275,369; 6,529,103; 6,643,903; 6,765,779; 6,765,780; 4,424,551; and 4,148,003, the contents of which are incorporated herein.
For high frequency EMI, such as that produced in the 450 MHz to 3000 MHz frequency range, it is very important that all of the leads that enter into and egress the AIMD be filtered. This is because once EMI enters into the housing of the AIMD, on even one lead, it can cross-couple or re-radiate to EMI sensitive adjacent circuits inside the pacemaker, implantable cardioverter defibrillator or the like. Accordingly, said EMI could find its way into a pacemaker sense circuit and create a dangerous situation, such as inhibition of the device.
In the past, telemetry was accomplished by an embedded coil that was contained in the titanium housing of the AIMD. Since the titanium shield is not magnetic, it was very easy to pass low frequency telemetry signals through close coupling subcutaneously from an external coil. This so-called external coil was held in close proximity to the AIMD allowing the physician to communicate with the AIMD. In this way, the physician could check battery status, recover patient waveforms, and also accomplish reprogramming and resetting.
However, a major market trend is that physicians and patients want to recover more and more stored data. That is, if a particular pacemaker patient is engaging in a sport activity and feels discomfort or what appears to be arrhythmias, it is desirable that the patient be able to return to the physician's office, even a week or two later, and recover the ECG waveforms from that time period. Accordingly, there is a need for more bandwidth and more stored data within the AIMD. Recent advancements in microchips allow for the storage of a great deal of data which can later be retrieved by the physician.
Another trend is that it is often inconvenient to close couple to an AIMD inside the patient. Putting a telemetry wand (head) immediately over the patient's chest and moving it around until one gets good communication with the AIMD or pacemaker is often problematic. Also, in a hospital setting it would be highly desirable to simply have a telemetry device anywhere in the patient's room that could continuously monitor and/or communicate with the pacemaker. Accordingly, there is a trend towards higher frequency (HF) distance telemetry.
In order to accomplish distance telemetry, the telemetry signals have to be at a higher frequency. In the past, the embedded coils operated in the kilohertz region with most of these between 50 and 140 kHz. For distance telemetry, there is typically an RF pin which egresses the hermetic terminal of the AIMD and sits in the device header block on the body fluid side of the AIMD housing. The device header block is typically a molded plastic or similar material. This RF pin is not connected through lead wires to body tissue. It sits in place and acts as a simple short stub antenna. The U.S. Federal Communications Commission has allocated a frequency range for such purposes called the MICCS frequency range, which is in the 400 to 405 MHz range. There are also higher frequency ranges in use around 800 to 900 MHz (or even up to 3 GHz). Advantages of such high frequencies are their relatively short wave length and efficient coupling to such a short antenna. This eliminates the need for a bulky embedded coil which was previously used inside AIMDs or in some cases it was so large that it had to be implanted external to the AIMD.
Another advantage of distance telemetry is that the band width and therefore the communication speed is much greater. That is, the physician can retrieve a great deal more data and at a faster rate than in the previous kilohertz frequency range telemetry transmissions. Accordingly, patient ECGs and other information can be readily displayed. In addition, since the higher frequency energy couples efficiently with the short RF pin antenna, it is no longer necessary to have an external telemetry head coil that is placed in close proximity to the patient's chest. That is, from a considerable distance, for example across the room, the physician can use a high frequency external programmer or an external reader and communicate with the implanted medical device.
All of this presents a problem, however, for EMI shielding within the AIMD. On the one hand, it is not possible to attach any of the prior art EMI filtered feedthrough capacitor(s) to this RF pin. The reason for this is that the broadband feedthrough capacitor filter is so efficient that it would remove the high frequency carrier along with the modulation of the telemetry signals. On the other hand, a significant problem arises when there is an unfiltered pin that ingresses and egresses the implantable medical device. It has been demonstrated through both coupling theory and laboratory testing that having an unfiltered pin enter the implantable medical device in close proximity to filtered pins can be problematic. That is, cross coupling can occur either through distributed capacitance, mutual inductive coupling or antenna propagation (re-radiation) between the unfiltered RF pin and the adjacent filtered lead wires (or directly to other circuits), such as those that may go to pacemaker sense circuits. Accordingly, having an unfiltered pin pass close to the filtered pins into the AIMD can significantly degrade the overall attenuation and shielding to electromagnetic interference.
FIG. 1 is an isometric drawing of a prior art active implantable medical device (AIMD) 30 such as a cardiac pacemaker. Referring to FIG. 1, one can see that there is a conventional titanium housing 32 which encloses and hermetically seals the electronics of the AIMD 30. There is also a header block assembly 34 into which lead wires 36 in accordance with ISO standards IS1, DF1 or the like are included. Also shown is an example of how the physician can plug in a mating plug 38 and cardiac lead 39 into the lead wire 36 assembly. In typical prior art applications, this would allow the physician to plug in lead wires 39 that are designed, for example, to be placed into the chambers of the heart 40. Referring back to FIG. 1, one can see an RF telemetry pin 42 which extends through the header or terminal 44 of the AIMD 30. One will notice that the RF telemetry pin 42 is not connected to any of the lead wires 36 within the header block 34 itself. The RF telemetry pin 42 forms a short stub antenna which is designed to communicate with an external transmitter used by a physician or other hospital personnel. The length of the RF telemetry pin 42 is important in that it should match a fraction of the wave length of the transmitted signal. According to the prior art, RF telemetry pin 42 could be associated with an overall hermetic seal or it can be installed with its own discrete hermetic seal.
FIG. 2 is a sectional view taken generally along line of 2-2 of FIG. 1. FIG. 2 shows the RF telemetry pin 42 in relation to the cardiac pacing and sensing lead wires 36. Also shown is a prior art feedthrough capacitor 46 which has been generally described by U.S. Pat. Nos. 4,424,551; 5,333,095; 5,905,627; 6,765,779 and the like. The AIMD 30 may be exposed to electromagnetic interference (EMI). These EMI signals impinge upon the housing 32. When the housing is metallic such as titanium the EMI is reflected off of or absorbed by the shielding material (shown as 48a). EMI may also couple into the lead wires 36 of the AIMD 30 that are designed to connect to body tissue (shown as 48b). However, the presence of feedthrough capacitor 46 generally decouples or shorts such EMI to the housing 32 of the AIMD 30 where it is dissipated as harmless heat energy. This prevents such EMI from entering into the inside of the AIMD 30 via the lead wires 36 and disrupting sensitive electronics. However, EMI may also couple to the RF telemetry pin 42. In other words, in addition to coupling to desirable RF telemetry signals, the RF telemetry pin 42 also picks up stray EMI signals (shown as 48c). These EMI signals are readily couple to the inside of the AIMD 30 (shown as 48d) where they can re-radiate to sensitive electronics such as the pacemaker sense circuits. A concern is that such EMI could be sensed as a normal cardiac rhythm which could cause, for example, a pacemaker to inhibit.
Accordingly, there is a need for an unfiltered but shielded RF telemetry pin, which is designed in such a way that it does not cross-couple, re-radiate or otherwise degrade the attenuation of adjacent filtered circuits. Additionally, there is a need for a methodology of shielding said RF pin wiring against such EMI so that the EMI cannot re-radiate or couple to sensitive AIMD electronics that are inside the AIMD housing. The present invention fulfills these needs and provides other related advantages.